Conventional thermostatic control devices used in home ovens and other thermal devices typically have a controller consisting of a bulb located within the heated area, a bellows or other operator located outside the heated area and a capillary connecting the bulb and operator. The entire system, bulb, capillary and operator, is filled with a thermally responsive material so that as the bulb is exposed to heat, the material expands and thus moves the bellows or operator. Temperature control within the oven or heated area is obtained by linking the operator to a switch or valve so that the source of heat is turned off when the operator has moved a predetermined amount. The system works very well as long as the bulb, capillary and bellows or operator remain pressure-tight, so that an increase in volume of the thermally responsive material results in the movement of the bellows or operator. However, occasionally a leak will develop in the system with the result that the thermally responsive material is permitted to leak out. When this occurs, an increase in heat in the area of the bulb does not cause a change in the position of the bellows or operator. In other instances, such as stuck or welded contacts, the source of heat may not be disconnected even though the bellows or operator has moved an amount normally sufficient to disconnect it. In either event, there is no control to interrupt the source of heat and a severe overheat situation may result. In extreme cases, this may cause a fire, or at a minimum, may cause the burning of whatever material is contained in the oven or other heating appliance.
This problem has been addressed in the prior art and in particular, in U.S. Pat. No. 4,691,097. This prior art discloses a second control device attached to the capillary between the temperature sensing bulb and the bellows or operator. This second control device protects against an overheat situation in the event of a loss of the thermally responsive material or in the event the contacts in the control device become stuck or otherwise permanently closed or inoperable. In the first instance, the volume of the thermally responsive material in the second control device is reduced to nothing or at least below a minimum. In the later situation, the volume in the second control device expands beyond a predetermined volume, generally somewhat beyond the normal operating range of the first control device. In either situation, the second control device terminates the heating of the heated area by the operation of switches to open a circuit or the closing of a valve or other control device.
It is therefore an object of this invention to provide a fail safe control device which will prevent an overheat situation when the integrity of the closed system is lost thereby resulting in a loss of the thermally responsive material.
It is another object of the invention to provide a fail safe control device which will prevent either initial heating or a further increase in heating when the closed system of the temperature control device is breached.
A further object of this invention is to provide a fail safe control device which will operate in the event of loss of the thermally responsive material below a predetermined minimum.
It is an additional object of the invention to provide a fail safe control device which utilizes a single pressure responsive control device, bellows or operator.
Other objects and features will be in part apparent and in part pointed out hereinafter.
The invention accordingly comprises the product hereinafter described, the scope of the invention being indicated in the following claims.